There are known various forms of disk driving device configured to record and/or reproduce information by rotating a magnetic recording medium in the form of a disk (hereinafter called a magnetic disk), for example. Among others, a disk driving device also called "hard disk device" is particularly used in a small-scaled, large-capacity system. Such a hard disk device is configured to rotate at a high revolution a magnetic disk which is made of a disk-shaped hard material having magnetic recording layers on surfaces thereof, and a magnetic head is opposed to the surface of the magnetic disk to effect signal recording or reproduction.
FIG. 27 shows one form of the disk driving device of this type. The disk driving device generally comprises a magnetic disk 1 on which information is recorded, a magnetic head 2 which records or reproduces information on or from the magnetic disk 1, a direct drive motor (not shown. Hereinafter called "DD motor") which drives the magnetic disk 1, a head driving mechanism 4 which moves the magnetic head 2 to a predetermined track on the magnetic disk 1, a base plate 5 which supports a housing sealingly accepting therein the magnetic disk 1, the magnetic head 2 and other members, a printed board 6 on which a motor driving circuit, control circuit, etc. are printed, and a frame (not shown) which positions the printed board 6 on the base plate 5.
The illustrated magnetic disk device includes two magnetic disks 1. Each magnetic disk 1 has two recording surfaces on opposite planar surfaces thereof. Therefore, the illustrated disk mechanism includes four magnetic heads 2 associated with respective recording surfaces of the magnetic disks 1. The magnetic heads are mounted to a swing arm 8 of the head driving mechanism 4 by cantilever springs. The head driving mechanism 4 consists of the swing arm 8, a steel belt 9 mounted to a part of the swing arm 8, a pulley 10 on which an intermediate portion of the steel belt engages, and a stepping motor 11 which has a drive shaft 12 supporting the pulley 10 combined with the steel belt 9, so that when the stepping motor 11 is driven, the swing arm 8 swings about a pivot pin 8a thereof. The magnetic disks 1, magnetic heads 2, swing arm 8, steel belt 9 and pulley 10 are accepted in the casing which consists of the base plate 5 and a top cover (not shown). To establish an airtight sealing of the housing, gaskets are used at the contact between the base plate 5 and the top cover and at the mounting portion of the stepping motor 11. Further, magnetic fluid is applied around the shaft of the DD motor for the same purpose. The swing arm 8 is provided with a shutter 17 extending outwardly away from the magnetic heads 2. Nearer to an airtight chamber of the base plate 5 is provided a photo interrupter 18 serving as an outside sensor. The photo interrupter 18 defines an insertion path 18a which receives the shutter 17 loosely. In the prior art arrangement, when the magnetic head 2 reaches the zero track position at the outermost circumference, the shutter 17 blocks the light path provided in the insertion path 18a of the photo interrupter 18.
In the arrangement using the stepping motor 11 to transport the magnetic head 2, head positioning is difficult when the track density of the disk is increased. More specifically, since different materials in the hard disk apparatus have different expansion coefficients, there occurs a problem called "thermal off-track" in which the position of the magnetic head 2 relative to the tracks varies with temperature. Therefore, in a 5.25 inch-type hard disk apparatus, it is difficult to precisely position the magnetic head 2 beyond 400TPI unless a servo system is used.
U.S. Pat. No. Re. 32,075 discloses an invention of a servo-control system. The system uses a data-masked servo sector including track center line servo-control data detected by a head to fix the position of the head according to one piece of servo information per one revolution of a magnetic disk. Since this servo-control system invites a decrease in the data recording length by an amount corresponding to the servo information, it is configured to slightly slow down the revolution to adjust the head transport speed. This arrangement, however, sometimes invites an instable movement of the head and sometimes increases the error rate.
U.S. Pat. No. 4,122,503 discloses another control system using a servo system in which the inner-most and outer-most tracks are used as particular servo tracks. This system is called "ID-OD system" in abbreviation of "inner diameter" and "outer diameter". In this system, the disk apparatus is configured to first read the outer servo track and effect a fine adjustment to place the head at the center of the track. Subsequently, the head is moved toward the inner servo track. In this operation, the step pulses of the stepping motor in the head driving mechanism are counted, so that when the head reaches the inner servo track, the head positioning mechanism effects a precise positioning to place the head at the center of the track. While the precise positioning is effected for each servo track, the positioning mechanism is informed of a correction amount necessary for finding the center of the track. Obtaining the correction amount, the positioning mechanism is enabled to correct precise positions of respective tracks according to information about the number of step pulses required for movement between the outer and inner tracks and the fine step correction amount required in each servo track.
However, since the positioning mechanism in this system must repeat the correction process on every occurrence of a positioning error, it takes much time not directly contributing to recording and reproducing operation when the positioning error often occurs, and decreases the utility of the disk driving apparatus. Further, since the mechanism does not include any means for re-adjusting the head once positioned on the data track, it cannot ensure that the head is maintained at a proper position.